A subject of the present invention is an apparatus for choking the control stage of a steam turbine.
According to a further aspect, a subject of the present invention is a steam turbine, especially for geothermal applications.
As is known, geothermal power stations exploit the endogenous heat of the earth to produce the steam that will directly or indirectly feed the steam turbine.
In particular, various cycles may be adopted for using the abovementioned steam, which cycles in the present case may be summarized as follows: turbines fed with indirect steam produced by means of heat exchangers, turbines fed with steam originating directly from the geothermal well, and finally turbines fed with what is known as xe2x80x9cflashedxe2x80x9d steam, meaning turbines in which the steam originating from the well is introduced into a water tank from which the steam for feeding the turbine is taken.
In more detail, the present invention relates to cycles of the second type or turbines fed with steam originating directly from the geothermal well. In such cases, the geothermal well is fed with water to achieve the necessary production of steam, a balance always being preserved between the conflicting requirements of feeding the turbines and xe2x80x9ccultivatingxe2x80x9d the geothermal well. In such conditions, it is clear that the production of steam from the subsoil does not guarantee a flow rate and a pressure that are constant over time. Specifically, if the flow rate is kept constant, for example, the pressure may vary substantially in the course of a few months, with the consequence that on any change in pressure the sections of the steam inlet into the nozzles of the turbine would also have to be changed.
For the purposes of sizing the inlet sections appropriately, therefore, the pressure and flow-rate conditions of the steam entering the turbine can only be estimated and it will subsequently be necessary to adapt the abovementioned inlet sections in order to obtain the optimum performance.
In addition to the abovementioned variations in flow rate and/or pressure inherent in the steam generation system itself, the use of direct steam also influences the said values, thus introducing a further degree of uncertainty into the data for designing the inlet sections of the turbine.
The steam originating directly from the geothermal well contains impurities, such as for example sulphur compounds or silicates, which may create scaling that forms on the inside walls of the turbine and/or cause wear and erosion of the steam passage sections. This phenomenon changes the internal geometry of the turbine and consequently the pressure and flow-rate values.
It is thus clear that, in order to be able to adapt the inlet sections of the turbine to the requirements resulting from the variability of the inlet flow rate and/or pressure, there is a very evident need to change the number of nozzles of the first stage through which the steam passes, increasing it when the pressure falls and vice versa.
For this purpose, it is known to shut down the turbine, gain access to the inner space and directly modify the inlet section of the nozzles, blocking off the necessary number.
However, it is clear that such a solution suffers from numerous disadvantages, primarily unacceptable machine shutdowns due not only to the time required for the work of blocking off the nozzles but also to the time necessary for cooling and restarting the turbine.
It is also known to make such an adjustment in a different manner that makes it possible to operate during the functioning of the turbine and hence to eliminate the abovementioned disadvantages. This arrangement provides for the use of valves fitted to the intake pipe admitting the steam into the turbine.
This solution also is not free from disadvantages, owing to the fact that the valves have to be fitted to individual pipes that feed groups of nozzles. In this case, specifically, the production of the pipes is complex and costly, and has an adverse effect on the optimum geometry of the intake pipe, inevitably causing additional losses.
The problem on which the present invention is based is therefore that of proposing an apparatus for the choking of the control stage of a steam turbine, and a steam turbine that possesses structural and functional characteristics such as to satisfy the abovementioned requirements and, at the same time, to overcome the disadvantages cited with reference to the known art.
This object is achieved by means of an apparatus for the choking of the control stage of a steam turbine and a steam turbine in accordance with, respectively, claim 1 and claim 17.